(1) Field of the Invention
The present invention relates to a phased array comprising multiple antenna elements or patches and, more particularly, to a broadband patch antenna having overlapping bands of many staggered tuned narrow bands made up of stacked patches.
(2) Description of the Prior Art
Phased array antennas are well known and are comprised of separate antennas or antenna elements, such as patches related to the present invention, which are excited directly or parasitically, at possibly different phases with respect to each other. The radiation pattern changes according to the phase of the excitation. The terms "patches" and "antenna elements" are used herein in an interchangeable manner. Similarly, the terms "indirect feed" and "parasitic feed" are used in an interchangeable manner and the elements associated with each thereof being sometimes referred to as "parasitic elements."
Antennas comprising patch elements are known in the art and some of which are described in the following patents: U.S. Pat. No. 5,003,318 to Berneking et al.; U.S. Pat. No. 5,043,738 to Shapiro et al.; U.S. Pat. No. 5,153,600 to Metzler et al.; U.S. Pat. No. 5,155,493 to Thursby et al. and U.S. Pat. No. 5,307,075 of Huynh, all of which are herein incorporated by reference. The antennas are comprised of single or multiple patches or antenna elements each typically having a square shape and each dimensioned so that the square patch resonates at a desired resonant frequency. The resonant frequencies of the patches are selected so as to cover a band of interest comprising the desired bandwidth of the antenna.
Single square patch antennas commonly possess a narrow band (5%) bandwidth. Multiple stacked patches making up an antenna array are known and one of which is a dual patch, dual band, dual fed stacked patch shown herein in FIG. 1 for an antenna 100 having an RF feed, such as a coaxial cable with an inner and outer conductor, with the inner conductor 102 connected to two patches 104 and 106. This antenna 100 of the two patches 104 and 106 has been attempted to be made to resonate at two frequencies of narrow bandwidth with both patches 104 and 106 being excited or fed in phase, that is, excited with the same phase of the applied excitation signal. Though feeding two elements of an antenna in phase exists for dipole and quadrifilar helix antennas, this configuration has been found not to work for the patch antenna.
A dual patch, dual band, top patch fed, parasitic bottom patch, stacked patch antenna is shown in FIG. 2 as antenna 108 which is also shown on p. 321, of "Handbook of Microstrip Antennas," edited by J. R. James & P. S. Hall, 1989. FIG. 2 shows the antenna 108 comprised of patches 104 and 106 with patch 104 being connected to the inner conductor 102 of the RF feed. This antenna 108 consists of the two patches 104 and 106 made to resonate at two frequencies of narrow bandwidth. Only the top patch 104 is fed; the bottom patch 106 obtains power by coupling to the top patch 104. The coupling involved for patch 106 is commonly referred to as parasitic because the element is not directly driven but, rather, is excited by energy radiated by another patch.
A dual patch, overlapping dual band, bottom patch fed, parasitic top patch, stacked patch antenna is shown in FIG. 3 as antenna 110 which is also described on p. 321 of "Handbook of Microstrip Antennas," edited by J. R. James and P. S. Hall, 1989. FIG. 3 shows the antenna 110 comprised of patches 104 and 106 with patch 106 connected to the inner conductor 102. The antenna 110 consists of the two patches 104 and 106 made to resonate at two frequencies whose individual bands overlap to form a broader band antenna. For example, bandwidths of 10-20% are possible for antenna 110. Only the bottom patch 106 is fed; the top patch 104 obtains power by coupling to the bottom patch 104.
Only antenna 110 allows broadbanding by combining the bandwidths of two individual patches or antenna elements. The band overlapping is done by direct feeding of one element at one resonant frequency and having another parasitic element resonating at a nearby frequency. In principle, additional parasitic patches can be added to make the antenna more broadband, although the practical prior art limit is three total elements. This is because the parasites can only place resonance/antiresonance loops in the impedance locus of the fed element and which may be further described with reference to FIGS. 4(A)-4(D), which illustrate the resultant impedances of a plurality of antenna elements.
FIG. 4(A) illustrates an impedance locus of one element at resonance. FIG. 4(B) illustrates an impedance locus of one direct fed element and two parasitic elements. FIG. 4(C) illustrates an impedance locus of the configuration of FIG. 4(B) with a third parasitic element added thereto. FIG. 4(D) illustrates an impedance locus of one direct fed element at antiresonance and two parasitic elements. More particularly, FIG. 4(A) illustrates a first resonant frequency .function.o of a single element antenna and the characteristic impedance Zo of the feed line of FIGS. 1-3. FIG. 4(B) illustrates the resonance of the element of FIG. 4(A) and second and third resonant frequencies .function.o.sub.2 and .function.o.sub.3 introduced by adding two elements to the antenna of FIG. 4(A), as well as a circular Voltage Standing Wave Ratio (VSWR) (shown in phantom) equal to approximately two (2). The second and third resonant frequencies .function.o.sub.2 and .function.o.sub.3, as well as .function.o.sub.4 of FIG. 4(C), are represented by loops in FIG. 4. FIG. 4(C) illustrates the impedance of the three element antenna of FIG. 4B with the introduction of a fourth resonant frequency .function.o.sub.4 from a fourth introduced element. FIG. 4(D) illustrates the impedance of the same three element antenna of FIG. 4(B), but with a different relationship therebetween and without the circle of VSWR of 2.
From FIG. 4(A) it may be seen that the locus of the fed element is a resonance (.function.o) passing through the feed Zo. A parasitic resonance of higher resonant frequency (.function.o.sub.2) can be used to place a loop slightly above Zo as seen in FIG. 4(B). As further seen in FIG. 4(B), another parasitic resonance of lower resonant frequency (.function.o.sub.3) can be used to place a loop slightly below Zo. This allows a broadband antenna that is matched about Zo as seen in FIG. 4(B). As shown in FIG. 4(C), if another parasitic element is added to resonate at a resonant frequency higher than .function.o.sub.2 (.function.o.sub.4), it introduces a loop in the locus of the fed element appreciably away from Zo and does not contribute to the impedance broadbanding; that is, the loop is shown outside a VSWR=2 circle in FIG. 4(C). The same effect (not shown) can be accomplished for a parasitic element added to resonate at a resonant frequency lower than .function.o.sub.3. From FIG. 4(C) it is seen that the practical limit of the number of antenna elements, some being parasitically fed, involved in broadband antennas with closely spaced elements is three (3).
As opposed to the locus seen in FIG. 4(A), for some patches, the impedance locus for the fed element is actually an antiresonance locus that passes through or near Zo. However, the same principle of adding loops to its locus with parasitic elements still applies, as shown in FIG. 4(D). With each addition of a parasitic element resonance loop to the antiresonance locus, frequencies increase as the locus rotates down the Smith Chart, whereas the resonance locus frequencies increase as the locus rotates up the Smith Chart. Furthermore, as with any phased array arrangement, care must be used in constructing antennas with parasitic elements because it is possible for currents to flow in the wrong direction on a parasitic element at some frequencies which can cause degradation of the desired radiation pattern generated by the phased array. It is desired that a phased antenna array be provided comprised of patches or antenna elements that is not limited to the usage of three patches, one of which is an indirectly fed parasitic element, but rather has patches of a number greater than two and all of which are directly fed.